Positive displacement pump having piston and/or liner with vapor deposited polymer surface

ABSTRACT

A metering pump for providing relatively small, precise volumes of fluid is provided. The pump piston and/or the interior of the cylinder has a surface finish exhibiting a roughness average within selected limits and a vapor deposited polymer such as polytetrafluoroethylene. A drive mechanism is provided for simultaneously rotating and reciprocating the piston within a cylindrical chamber. Fluid is thereby drawn into the chamber and expelled.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The field of the invention relates to metering pumps for pumpingrelatively precise volumes of fluid.

[0003] 2. Brief Description of the Prior Art

[0004] Positive displacement metering pumps and dispensers are used insingle-shot or multiple-dispense and continuous-dispensing applicationfor a variety of industries. Some examples are medical diagnostic,industrial, and agricultural to name a few. U.S. Pat. Nos. 3,168,872,5,020,980 and 5,015,157 disclose positive displacement pumps withvarious means for rotating and reciprocating a piston and adjustingpiston stroke length. These patents are incorporated by referenceherein. While some positive displacement pumps have pistons that arecaused to rotate continuously about their longitudinal axes as they arereciprocated, other such pumps have pistons that oscillate back andforth about their longitudinal axes while being caused to reciprocatealong their axes. These pumps are sometimes referred to as reciprocatingoscillating pumps.

[0005] Precision positive displacement metering pumps and dispensers areused to accurately control the movement or transport of fluids through afluidic system as needed. Such pumps and dispensers are used when verysmall volumes of a given fluid are to be transported with a very smallmargin of error relative to volumetric accuracy and precision. (The term“pump” is generally employed in applications where fluid is transportedin a substantially continuous manner. The term “dispenser” is used inapplications where fluid is dispensed on demand or at selectedintervals. For the purposes of this patent application, the terms “pump”and “dispenser” are used interchangeably.)

[0006] One problem with prior metering pumps and dispensers when movingcrystal-forming aggressive solutions is the pump/piston assembly has thepotential to seize, i.e., get stuck and/or freeze. In worst caseconditions, under intermittent use, pumps sit dry and solids are formedin the pumping chamber, creating a seize condition. In order to preventa seize, pumps should be kept wet at all times whenever possible, and beflushed out in between runs as well as during any down or idle time.Metering pumps typically have very tight tolerance mating components andcompletely flushing out a system is very difficult.

[0007] Another approach employed to prevent a seize condition is tocreate a smooth-surface-profile on the piston O.D., the sliding surface.The smooth-surface-profile could prevent crystal formations on thesurface substrate. The super-smooth surface could reduce the incidenceof crystals drying and attaching to the piston O.D. substrate andultimately causing a freeze condition. A problem associated with thesuper-finish approach is a seal-weepage condition. The super-finishcondition may further prevent proper pump seal burnishing or wear-in. Ifa surface profile is too smooth, a slip-stick condition occurs, i.e., nomaterial transfer from the seal to the mating seal surface. This isessential for proper seal operation. Pump seal manufacturers recommend aspecific seal surface profile. The smooth finish causes a seepagecondition. The seepage condition does not directly affect the pump'sperformance but it does affect overall product reliability. Fluids thatseep out over time dry up, forming crystals that assist in causing sealfailures. In addition to this, the seepage can attack the external pumpcomponents over time. In order to prevent this, pump users employpreventative maintenance procedures, spraying the assembly with acleaning solution. However, the external pump components are thenattacked by the cleaning media, which may be corrosive and potentiallydamaging to the pump body.

[0008] Another problem associated with the super-finish approach is pumpperformance relative to precision and accuracy, and siphoning effects.Precision positive displacement pumps and dispensers requirevery-tight-tolerance mating parts to assure proper operation. With asmooth-finish surface profile, the mean roughness as well aspeak-to-valley ratios are greatly reduced. This condition significantlyincreases the pump component clearances which in turn increasessiphoning through the pump. Minimal siphoning is an important feature ofpositive displacement metering pumps and dispensers.

[0009] Another problem associated with metering pumps relative to pistonsurface profiles are the microscopic imperfections on the pistonsurface. When a pump is moving a given fluid in either a dispense- orcontinuous-mode, the fluid will be subjected to internal tangentialshear stresses. These stresses act along the surface of the pistonparallel to the surface. This causes system friction at the pistonsurface. Ideally the fluid flow-path through the pumping chamber islaminar, but turbulence created by microscopic imperfections on thesurface profile enhances these imperfections. These imperfections tripthe smooth laminar flow into an unpredictable turbulent flow which inturn causes pump system inaccuracies relative to precision and accuracy.In order to reduce this effect, pump users typically add a surfactant tothe fluid media. This assists in creating a hydroscopic, lubricoussurface.

[0010] It would be desirable to have a pump/seal combination that hasreduced incidents of seize, stuck or freeze condition, a reduced sealseepage as well as reduced siphoning, and improved flow characteristics.

SUMMARY OF THE INVENTION

[0011] A positive displacement pump is provided that comprises a housingthat includes a cylindrical working chamber and a piston positioned atleast partially within the working chamber. The piston includes a duct.Two or more passages extend through the housing and are in fluidcommunication with the working chamber. The passages are alsocommunicable with the duct depending on the position of the piston. Adrive mechanism(s) is coupled to the piston for rotating andreciprocating the piston, thereby causing fluid to be drawn into theworking chamber through one or more of the passages and pumped outthrough another one or more of the passages. Piston rotation can bethrough 360° or, in the case of a reciprocating oscillating pump,through a smaller arc. The surface of the piston includes a vapordeposited polymer, preferably polytetrafluoroethylene (PTFE). While notrequired, the surface of the working chamber may also include a vapordeposited polymer.

[0012] The surface of the piston preferably has a degree of roughnessprior to application of the polymer. In other words, the super-finishapproach discussed above is not employed. The combination of a rougherfinish and a vapor deposited polymer coating provides a superior productthat will prevent seize conditions. Moreover, it will allow quick “burnin” when the pump or dispenser is new. The surface of the pistonpreferably comprises a dimensionally stable material such as a ceramicor ceramic-type material, though other dimensionally stable materialssuch as aluminum and stainless steel that can be treated with a vapordeposited friction-reducing material could possibly be used at least inselected applications. In a preferred embodiment of the invention, theouter surface of the piston has a finish with a roughness average Ra ofat least about four microinches (μin) and preferably at least abouteight microinches, the average maximum height of the preferred surfaceprofile Rz being about fifty μin. The average Ra is preferably in arange of about four to sixteen μin, and more preferably eight to sixteenμin.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is an exploded elevation view of a metering pump accordingto the invention;

[0014]FIG. 2A is an enlarged sectional side elevation view of thepiston/cylinder assembly shown in FIG. 1;

[0015]FIG. 2B is a greatly enlarged sectional view thereof;

[0016]FIG. 3A is an enlarged sectional side elevation view of apiston/cylinder assembly according to a second embodiment of theinvention; and

[0017]FIG. 3B is a greatly enlarged sectional view thereof.

DETAILED DESCRIPTION OF THE INVENTION

[0018] While this invention is satisfied by embodiments in manydifferent forms, there are shown in the drawings and will be hereindescribed in detail preferred embodiments of the invention with theunderstanding that the present disclosure is to be considered exemplaryof the principles of the invention and is not intended to limit thescope of the invention to the embodiments illustrated. The scope of theinvention will be measured by the appended claims and their equivalents.

[0019] Referring to FIG. 1, an exploded view of a piston/cylinderassembly for a metering pump is shown. A cylinder housing 10 having anexternally threaded end 12 defines a cylindrical chamber. A pair ofopenings 14 are provided in the housing in diametrically opposedrelation. The housing is preferably made from a material that providesgood chemical and temperature resistance and good mechanical strength,such as ethylene-tetrafluoroethylene. A cylindrical liner 16 is mountedwithin the housing 10. The liner includes a pair of diametricallyopposed openings 18 that are aligned with the openings 14 in the housing10. The liner defines the working chamber of the metering pump. It willbe appreciated that the housing and liner may each have more than twoopenings. The housing and liner may be combined as an integral structuremade of the same material in some applications.

[0020] A piston assembly 20 including a gland nut 22 and piston 24 areadapted for coupling to the housing 10. The piston 24 includes acylindrical body and a duct in the form of a flat 26 formed at one endthereof. A typical piston diameter may be about one quarter to one halfinch, though diameters outside this range are used. A channel mayalternatively be formed in the piston instead of a flat. The totaldiametrical clearance between the cylindrical portion of the piston andthe liner 16 is preferably between 0.000050 and 0.000100 inches. A pin28 extends radially from the end of the piston 24 on the opposite sideof the gland nut 22. The pin is used for rotating the piston 24 as wellas causing it to reciprocate within the pump working chamber. Variousmechanisms are known for connection to the pin and providing adjustmentsin pump speed and piston stroke, such as disclosed in U.S. Pat. Nos.3,168,872, 5,020,980 and 5,015,157. A preferred mechanism includes adrive cylinder 40 coupled to the drive shaft 42 of a motor 44. Suchcoupling can be effected by a lock screw 46. A ball and socket fitting48 is mounted to the drive cylinder, and receives the end of the pin 28.The length of the stroke is determined by the angle formed between thelongitudinal axes of the piston 24 and drive cylinder 40. Another drivemechanism (not shown) involves the use of a drive member including asinusoidal groove in which the pin is positioned. The piston movesaxially back and forth within the liner 16 as the pin is caused totravel through the groove. As discussed above, full rotation of thepiston is not required, and only limited rotation is indeed provided inreciprocating oscillating pumps. In operation, fluid is drawn into theworking chamber through one set of aligned openings 14, 18 as the pistonmoves in a first axial direction and is expelled through the other setof aligned openings 14, 18 as the piston moves in the oppositedirection. Rotation of the piston causes the flat 26 to be orientedtowards one set of the aligned openings and then the other, allowingprecise volumes of fluid to be transported.

[0021] The gland nut 22 is internally threaded, and can be coupled tothe threaded end 12 of the housing 10. A gland washer 30 and a three lipseals 32 are provided for preventing fluid leakage. It will beunderstood that a cartridge seal or other type of sealing mechanism canbe employed. The particular materials chosen for the washer and sealsshould be compatible with the fluid to be pumped. A washer made frompolytetrafluoroethylene (PTFE) and seals made from a corrosion andwear-resistant material exhibiting low friction are acceptable for anumber of applications. PTFE-based materials such as RULON AR have beenused in the industry for sealing purposes, as well as virgin PTFE. Itwill be understood that the number of seals and their composition isapplication-specific, and that other arrangement for sealing the workingchamber could be employed in accordance with the invention.

[0022] The piston 24 and liner 16 are both preferably comprised of aceramic material such as alumina or zirconia. YTZP (Yttria TZP) is asuitable form of zirconia for a number of applications. ZTA (Zirconiatoughened alumina) is another acceptable material for these elements.

[0023] The finish on the piston 24 is important for optimal performanceof the pump. In combination with the finish, the presence of a vapordeposited polymer on the piston provides significant performancebenefits. Referring to FIG. 2A, the vapor deposited polymer 34 extendsfrom one end of the piston to a point outside the working chamberextending beyond the pump seal(s). The portions of the piston that willcontact fluid and the seals are accordingly provided with the vapordeposited polymer, preferably PTFE. Vapor deposition of the polymerensures that the polymer will not wear off the piston during use. Incontrast, polymers applied as coatings tend to wear over time. Inaddition to the resulting degradation of pump performance, a coatingthat wears off the piston of a metering pump or which otherwise degradeshas other potentially serious ramification in certain applications wherepositive displacement metering pumps are employed. For example, meteringpumps can be used in the manufacture of semiconductor wafers where theintroduction of any impurities from a wearing or degrading coating canseriously impair the quality of the finished product. Impurities mustalso be avoided where metering pumps are used for delivering precisequantities of fluid in medical applications or for scientific studies.

[0024] Referring to FIG. 2B, the finishes of both the piston 24 and theliner 16 have a roughness average Ra, an average maximum roughness Rzand a maximum profile depth Rm. (This figure is for illustrativepurposes, and may not be representative of the actual surface profilesof the piston and liner surfaces.) The surface finish of the piston issuch that it does not tend to wear out the seal(s) of the pump as itrotates and reciprocates, provides an acceptable surface for vapordeposition of the polymer, and allows the piston 24 to move smoothlywithin the liner 16. In this preferred embodiment, there is no polymerdeposited on the inner surface of the liner, and the piston and linerboth have a roughness average of at least about four and more preferablyat least about eight but less than about sixteen microinches. Thepreferred average maximum surface roughness Rz is about fiftymicroinches. Maximum profile depth Rm is also preferably about fiftymicroinches. The combination of surface finishes of proper roughness, avapor deposited polymer such as PTFE, and piston/liner clearance in theappropriate range provide a pump or dispenser that is highly suitablefor delivering precise amounts of fluid in a reliable manner. Thesurface finishes of one or both the piston and liner could possiblyexceed a roughness average of sixteen microinches provided that theseelements do not then tend to bind. The maximum roughness of eitherelement would be likely to vary depending on the roughness of thesurface finish of the other element it engages, the materials from whichthe elements are manufactured and the materials comprising the seals.Roughness averages of twenty or even up to twenty-four microinches maybe possible using the ceramic materials discussed above, though anaverage of less than sixteen has proven reliable in ensuringsatisfactory performance when Rm and Rz are also within the abovelimits.

[0025] In the alternate embodiment shown schematically in FIGS. 3A and3B, both the piston and liner are made of a ceramic or ceramic-typematerial, such as alumina or zirconia, and have surface finishes asdescribed with respect to the embodiment of FIGS. 2A and 2B. In thisembodiment, both the inner surface of the liner 16 and the outer surfaceof the piston 24 include a vapor deposited polymer 34. The polymerpreferably comprises PTFE, and more preferably is substantially purePTFE, like the polymer on the piston 24 shown in FIGS. 2A and 2B. Athird possible alternative (not shown) could include a liner having avapor deposited polymer and a piston that includes no vapor depositedsubstance.

What is claimed is:
 1. A metering pump for dispensing small, precisevolumes of fluid, comprising: a pump housing; a chamber within said pumphousing, said chamber being bounded by a cylindrical wall within saidhousing; a piston having a cylindrical body portion extending withinsaid chamber, said cylindrical body portion of said piston having acylindrical outer surface and a duct formed therein; two or morepassages extending through said pump housing and communicating with saidchamber; a drive mechanism coupled to said piston for rotating andreciprocating said piston within said chamber; said outer surface ofsaid cylindrical body portion of said piston including a surface finishexhibiting a roughness average of at least about four microinches and avapor deposited polymer.
 2. A metering pump as described in claim 1,wherein said polymer comprises polytetrafluoroethylene.
 3. A meteringpump as described in claim 1, wherein said polymer consists essentiallyof polytetrafluoroethylene.
 4. A metering pump as described in claim 1,wherein the total diametrical clearance between said cylindrical outersurface of said piston and said cylindrical wall of said chamber isbetween about 0.000050 and 0.000100 inches.
 5. A metering pump asdescribed in claim 4, wherein said surface finish exhibits a roughnessaverage of at least about eight microinches.
 6. A metering pump asdescribed in claim 5, wherein said piston is comprised of a ceramicmaterial and said surface finish exhibits a roughness average of betweenabout eight and sixteen microinches.
 7. A metering pump as described inclaim 5, wherein said cylindrical wall includes a surface finishexhibiting a roughness average of at least about eight microinches.
 8. Ametering pump as described in claim 7, wherein said cylindrical wallincludes a surface finish exhibiting a roughness average of betweenabout eight and sixteen microinches.
 9. A metering pump as described inclaim 7, including a seal within said pump housing and engaging saidcylindrical body portion of said piston.
 10. A metering pump asdescribed in claim 9, including a ceramic liner within said housing,said ceramic liner bounding said chamber.
 11. A metering pump fordispensing small, precise volumes of fluid, comprising: a pistonincluding a substantially cylindrical outer surface having a surfacefinish exhibiting a roughness average of at least about four microinchesand a duct formed within said outer surface; a housing having an innersurface defining a cylindrical chamber, said piston extending into saidcylindrical chamber; an inlet passage extending through said housing; anoutlet passage extending through said housing; means for rotating saidpiston about a longitudinal axis; means for reciprocating said pistonwithin said chamber such that fluid is drawn into said chamber throughsaid inlet passage and expelled through said outlet passage; and a vapordeposited polymer on said substantially cylindrical outer surface ofsaid piston.
 12. A metering pump as described in claim 11 wherein saidpolymer comprises polytetrafluoroethylene.
 13. A metering pump asdescribed in claim 11 wherein said polymer consists essentially ofpolytetrafluoroethylene.
 14. A metering pump as described in claim 11wherein the total diametrical clearance between said outer surface ofsaid piston and said inner surface of said housing is between about0.000050 and 0.000100 inches.
 15. A metering pump as described in claim14 wherein said surface finish of said piston exhibits a roughnessaverage between about eight and sixteen microinches.
 16. A metering pumpas described in claim 14 wherein said inner surface of said housingexhibits a roughness average of at least about four microinches.
 17. Ametering pump as described in claim 16 wherein said inner surface ofsaid housing and said outer surface of said piston are comprised ofceramic materials.
 18. A piston and cylinder assembly for a meteringpump for dispensing small, precise volumes of fluid, comprising: aceramic piston including a substantially cylindrical outer surfacehaving a surface finish exhibiting a roughness average of at least aboutfour microinches and a duct formed within said surface; a housing havinga ceramic inner surface defining a cylindrical chamber and exhibiting aroughness average of at least about four microinches, said pistonextending within said chamber; a vapor deposited polymer comprisingpolytetrafluoroethylene on at least one of said substantiallycylindrical outer surface of said piston and said inner surface of saidhousing.
 19. The assembly of claim 18 wherein the total diametricalclearance between said cylindrical outer surface of said piston and saidinner surface of said housing is between about 0.000050 and 0.000100inches.
 20. The assembly of claim 18 including a seal mounted to saidhousing and engaging said cylindrical outer surface of said piston. 21.The assembly of claim 18 wherein said surface finish of said pistonexhibits a roughness average of at least about eight microinches andsaid polymer is on said outer surface of said piston.
 22. The assemblyof claim 21 including an inlet passage extending through said housingand an outlet passage extending through said housing.
 23. The assemblyof claim 21 wherein said housing includes a ceramic liner mounted tosaid housing and defining said ceramic inner surface, and first andsecond openings in said liner communicating, respectively, with saidinlet and outlet passages.
 24. The assembly of claim 23 wherein saidroughness averages of said outer surface of said piston and said ceramicinner surface are less than about sixteen microinches.